But you may also want to consider an alias since this is substitution at the start of the command:

alias lsr='ls -rt'

This is, after all, how many systems give you the very handy "long ls", with something like:

alias ll='ls -al'

You'll just need to make sure this is present in something that's called for specific cases as needed, such as in .bashrc for interactive shells, or in a file sourced at the start of your script itself (and ensuring aliases are expanded with shopt -s expand_aliases). Those are two possibilities but there are no doubt others.

In the case at hand, the { in {ls is not recognized as a shell keyword, because it isn't recognized as a separate word due to immediately being followed by ls.

Bash therefore sees single word {ls, which is unexpected at this position, and fittingly reports syntax error near unexpected token '{ls'.

Note that the closing } does not need preceding whitespace (even though you may want to add it for visual symmetry), because it is preceded by metacharacter ;, which is by definition recognized as its "own thing", and thus implicitly causes the } to be recognized as its own word.

On a related note: Since the closing } is on the same line as the ls command, the ls command must be terminated with control operator ; - otherwise, the } would be interpreted as another argument to ls.